Editing Assertion (software development) (section)

== Details ==

The following code contains two assertions, <code>x > 0</code> and <code>x > 1</code>, and they are indeed true at the indicated points during execution:
<syntaxhighlight lang="c">
x = 1;
assert x > 0;
x++;
assert x > 1;
</syntaxhighlight>

Programmers can use assertions to help specify programs and to reason about program correctness. For example, a [[precondition]]—an assertion placed at the beginning of a section of code—determines the set of states under which the programmer expects the code to execute. A [[postcondition]]—placed at the end—describes the expected state at the end of execution. For example: <code>x > 0 { x++ } x > 1</code>.

The example above uses the notation for including assertions used by [[C. A. R. Hoare]] in his 1969 article.<ref>[[C. A. R. Hoare]], [http://lambda-the-ultimate.org/node/1912 An axiomatic basis for computer programming], ''[[Communications of the ACM]]'', 1969.</ref> That notation cannot be used in existing mainstream programming languages. However, programmers can include unchecked assertions using the [[Comment (computer programming)|comment feature]] of their programming language. For example, in [[C++]]:

<syntaxhighlight lang="c">
x = 5;
x = x + 1;
// {x > 1}
</syntaxhighlight>

The braces included in the comment help distinguish this use of a comment from other uses.

[[Library (computing)|Libraries]] may provide assertion features as well. For example, in C using [[glibc]] with C99 support:

<syntaxhighlight lang="c">
#include <assert.h>

int f(void)
{
    int x = 5;
    x = x + 1;
    assert(x > 1);
}
</syntaxhighlight>

Several modern programming languages include checked assertions – [[statement (programming)|statements]] that are checked at [[Run time (program lifecycle phase)|runtime]] or sometimes statically. If an assertion evaluates to false at runtime, an assertion failure results, which typically causes execution to abort.  This draws attention to the location at which the logical inconsistency is detected and can be preferable to the behaviour that would otherwise result.

The use of assertions helps the programmer design, develop, and reason about a program.